A method of demagnetization is

Analysis of the Question

The question asks for a valid method of demagnetization. Demagnetization is the process of removing the magnetic properties from a magnetized material (like a permanent magnet).

Concept Explanation

To demagnetize a magnet, you need to disrupt the alignment of the magnetic domains (the tiny regions within the material that act like small magnets). This is typically achieved by:

1. Heating: Heating a magnet to its "Curie temperature" causes the atoms to vibrate so violently that the aligned domains lose their orientation. If cooled in an East-West direction (away from the Earth's magnetic field), the domains remain in a random, non-magnetic state.
2. Electrical Method: Placing the magnet in a solenoid connected to an alternating current (AC) and slowly withdrawing it.
3. Mechanical Method: Hammering or dropping a magnet repeatedly, but this must be done in an East-West direction to avoid the Earth's magnetic field from re-aligning the domains.

Step-by-Step Solution

• Option A: Placing a magnet in a solenoid is used for magnetization (using DC) or demagnetization (using AC). However, the option is incomplete because it doesn't specify the use of AC and the withdrawal process.
• Option B: Double-stroking is a method used for magnetization, not demagnetization.
• Option C: Heating a magnetic bar to red heat gives the molecules enough energy to vibrate out of their aligned positions. By allowing it to cool in the East-West direction, you ensure that the Earth's magnetic field does not influence the domains to re-align, effectively demagnetizing the bar. This is a standard, scientifically accepted method.
• Option D: Hammering a magnet can demagnetize it, but it must be done in the East-West direction. Hammering in the North-South direction actually helps align the domains with the Earth's magnetic field, which can lead to magnetization rather than demagnetization.

Comparing these, Option C is the most accurate and reliable method described for demagnetization.

Conclusion

This is correct because heating to red heat destroys the magnetic domains, and cooling in the East-West direction prevents the Earth's magnetic field from re-magnetizing the bar.

Exam Tip

Always remember the "E-W" rule for demagnetization: If you are using mechanical methods like hammering or heating to demagnetize, you must always perform the action in the East-West direction. If you do it in the North-South direction, you are actually working with the Earth's magnetic field, which encourages magnetism!